Devices/apparatus for providing for global positioning/navigation using satellite systems require the ability to receive and decode a number of communication signals from a number of satellites. For the sake of convenience, the foregoing discussion and examples will be focussed around GPS and associated portable electronic devices (e.g. GPS receivers, portable communications devices, including mobile telephone and the like), but it will be appreciated that the invention is not limited to any particular GNSS or portable electronic devices.
During initiation of a positioning request, the device/receiver should be able to identify at least four visible satellites from data provided by GPS signals being received from satellites. From the data provided from one or more satellites, the further visible satellites can be determined (i.e. using received almanac data, which can take in excess of 12 minutes to download).
As known, such devices/receivers rely on both almanac and ephemeris data in establishing a geographical position. It will be appreciated that almanac data provides course orbital parameters for all satellites in the GPS constellation. This data is not particularly accurate, but it usually remains valid for several months. In contrast, ephemeris data provides more precise orbital parameters that, together with clock corrections for each satellite, are required for precise positioning, e.g. for a 3D geographical position fix. Each satellite broadcasts its ephemeris data, which broadly speaking has a life span of approximately four hours.
As mentioned above, each identified satellite is able to provide precise ephemeris data for that particular satellite. This is used in conjunction with data relating to the time of transmission of a signal from that satellite to assist in providing for a geographical position of the device/receiver, or to assist in providing for a so-called fix.
Without having prior knowledge of the locations of particular satellites in relation to the device/receiver (e.g. knowledge of those satellites that should be in view to the device/receiver), the device/receiver must perform laborious acquisition processes of received signals in order to ascertain which particular satellites are available to provide for a first fix.
Having prior knowledge of the particular satellites that are visible from a particular position (e.g. visible in the overhead sky to a device/receiver on the surface of the earth), allows the device/receiver to quickly identify those particular satellites, and provide, or in other words determine, the precise geographical location of the device/receiver.
To determine which particular satellites are visible, the device/receiver has some knowledge of the satellite orbit data (e.g. Almanac and/or Ephemeris), an estimate of the Satellite system time and an approximate estimate of the receiver's location.
The listing or discussion of a prior-published document or any background in this specification should not necessarily be taken as an acknowledgement that the document or background is part of the state of the art or is common general knowledge. One or more aspects/embodiments of the present invention may or may not address one or more of the background issues.